1. Field of the invention
The present invention relates to an error correlation method for improving the quality of images photographed by an aerial vehicle using laser radar. More specifically, the present invention relates to the use of disclosed algorithms, which do not increase the system complexity and yet reduce the accumulation of in-track geometric distortion error during photographing. The present invention also discloses a fire table of equations used to improve the reproduction quality of the laser images by reducing tangential error accumulation in the cross-track direction during photographing.
2. Related Prior Art
The use of aerial vehicles to obtain aerial photographs of, for example, an object, a terrain or a plant, etc. on the ground is well known to individuals skilled in the art. Aerial laser photography is an essential part of commercial, military and economic information gathering activities. Aerial laser photography uses the fundamentals of “points”, “lines” and “planes” to obtain desired photographs. Aerial laser photographs are produced by performing in-track point by point scanning with instant, one-shot laser pulse using rotational scan lenses, while an aerial vehicle maintains relative speed and travels in an orthogonal direction to establish a two-dimensional photograph. Each pixel of an obtained laser photograph is sampled at a different time. The operational mechanism of laser photography is quite different from the operational mechanism of conventional, camera-based photography which operates as a one time exposure and collects pixels one by one to form the desired photographs.
Because geometric distortion occurs in aerial laser radar photographs due to differences in both the sampling timing and the length of the sampling timing related prior arts have suggested two approaches for reducing in-track geometric distortion of aerial laser radar photographs by uniformly distributing the pixels across the photographs. The first approach is based on controlling the rotational speed of the scanning lens of the laser photographic system while keeping the relative speed of the aerial vehicle constant. The second approach is based on maintaining the rotational speed of the scanning lens of the laser photographic system at the existing speed of the aerial vehicle.
In the first approach, the related prior art uses fixed rotational speed during scanning while keeping the aerial vehicle at a constant speed so as to obtain laser radar photographs based on optimum in-track scan resolution. This approach is not without fault. When the laser radar photographic system is photographing from an aerial vehicle at high acceleration and high altitude it is difficult to maintain constant speed for a long duration due to unavoidable turbulence and wind direction changes. Also, obtaining good quality photographs depend on the performance of the laser radar itself, the stability of the relative positioning of the laser radar and the area to be photographed. If the relative positioning of both the former (laser radar) and/or the latter (area) are not stable, the photographs obtained through the laser radar system will be geometrically distorted due to inconsistent scanning points and inconsistent lines reproduced. Aerial photographs with geometric distortions are distorted representation of the areas photographed and thus lack value.
In the second approach, the related prior art controls the driving motor of the rotational speed of the scanning lens of the laser photographic system at the existing speed of the aerial vehicle. This approach attempts to maintain the driving motor speed of the scanning lens and the aerial vehicle speed at the same speed levels when photographing. This approach is also flawed because the response time for the motor to adjust and stabilize its speed may take as long as several seconds, especially for motors not equipped with speed reduction mechanism. For example, motors that use only inertia to reduce its speed may take a longer time.
In contrast to the approaches of the related prior art, the present invention discloses novel algorithms, which do not increase the system complexity and yet reduce the accumulation of in-track geometric distortion error during photographing. The present invention also discloses a fire table of equations for improving the reproduction quality of laser images by reducing tangential error accumulation in the cross-track direction during photographing.